Manufacture of nitrocellulose



Patented July 10, 1934 I 1,965,577 MANUFACTURE OF mTRoonL p osE PaulB. Cochran, Parlin, N.-J., assignor to E. I.-

du Pont de Nemours' & Company; Wilmington, a Del., acorporation of Delaware w N Drawing. Application June 21,1933, 7

' Serial No. 676,923

Claims. (c1. 260- 145) This invention relates to improvements in the manufacture ofnitrocell'ulose and, more'p'artic 'ula'rly, to maintaining special conditions while the nitrocellulose is in'the acid wetstate whereby 5 the so-called grain in the solutions of the nitrocellulose may be substantially eliminated.

For many .years manufacturers of nitrocellulose have been bothered at various periods by their product in commercial; solutions being defficient in various degrees in qualitywith respect to 'grain. By grain those skilled in this art mean an appearance of grain in the nitrocellulose solutions and films deposited therefrom. This particular condition may be readily observed in more exaggerated cases by inverting a glass bottle-"containing the nitrocellulose solution. The grain will be evident as the solution runs down the side.

of the bottle. In very severe instances, settling of the grainy material to the bottomof the bottle out a quantityzof the solution on a spatula orknife blade and allowing it to form afan-shaped drip back into the bottle. By looking through the drip the amount of grain present may be readily observed. In generaLthe :grainy'portion-- has the appearance of material notentirely dissolved, i. e., transparent gel-likeparticles whichare decidedly harmful to the uniformity and smoothness of the solution. The grain in a nitro cellulose solution may be rather accurately described as giving an applesauce appearance to the solution. 7

As consumers became mor quality-conscious, the elimination of this undesirable characteristic of graining in nitrocellulose became a very serious consideration. It is particularly ob- 40 jectionable in nitrocellulose solutions designed for use in connection with finishing leather, especially patent leather. 'In the case of patent leather, grainy solutions give a'mottled surface rather than the perfectly smooth,high-gloss finish normally obtained. It is also of particular importance that nitrocellulose which is to beused for the'production' of photographic [film base be free of grain. It should beunderstood, however, that this graining characteristic is objectionable in practically all uses of nitrocellulose, although more particularly so in the above mentioned uses.

Since the graining characteristic of nitrocellulose appears to be more pronounced during the hot months of the year, the trouble seemed to be will give increased yields. is to provide a process which will prevent denitra- ,tion or drop-in itrogen content of the nitrocel connected with high temperaturesl, However, no consistent elimination of the. troublewas known until applicants entry into the field.

An object of the present invention .is to 'provide an economical and practically feasible processfforthe manufacture of nitrocelluldsewhich will insure a product having uniform absence of grain in solutions, regardless of atmospheric, conditions during the manufacture of the product. A'further object of the invention is to provide a process of manufacturing nitrocellulosev which A still further object lulose during. manufacture.

Another object of theinvention is toprovide a process whereby filtration losses of nitrocellulose are greatly reduced. Other objects of the inventionwill be apparent from the description given hereinafter.

The above objects areaccomplished according to thepresent iinvention by iriaintaining the ni- "trocellulos'e while inthe acid wet state in a dehumidifiedinert atmosphere, preferably dehumidified air., 7 I

In the preparation of nitrocellulose, cellulose in anyone of: numerous forms is:;treated with being subsequently drowned in water. In' order to remove the noxious fumes, such as nitrous oxide, et cetera, which are givenofi during the centrifuging. aforce draft of'air is drawn through the separator. Although the normal centrifuging period is only twoor three'minut'es, the volume of air passing through the separator is quite 1arge -I x i Applicant's invention is based-on the discovery that the objectionable graining characteristic of nitrocellulose is caused, not directly by high temperature, but by the nitrocellulose while in the acid wet state coming in contact with moist air.

Since saturated air at a high temperature contains much more moisture than saturated air at :a lower temperature, it is apparent that graining difficulties should be more pronounced during the "summer months than during the cold months of that, indirectly, temperature wouldbe apt to have some connectionwith graining difliculties,

since air at high temperatures is more ,likely to in each case was:-

time in which it is in the acid wet state is kept in a dehumidified atmosphere, but since the'volume of air drawn through the nitrocellulose during the centrifuging is so high,.and consequently, the opportunity for the nitrocellulose to absorb moisture during this step is so much greater in proportion than during the rest of the" handlingof the nitrocellulose before it .is drowned, it is suflicientjto merely dehumidify the g ls, applied to the centrifugal separator. I, o

In order tofillustratethe effectiveness of the present invention and to show that grain in nitrocellulose solutions may be'produce d or prevented at will bycontrollin'g the moisturecdntent of the air being drawn through the, centri- 'fugal separator during the separation of the nitrocellulose from the 'nitrating acids, several the acid fumes through an outlet in the side of the casing.

At the end of the wringing cycle the charge was quickly quenched and then stabilized by boiling in water followed by hot water washes in accordance with practice well known in the art. In the washing process and for the change of .water for the boiling treatment the nitrocellulose was poured into a Buchner funnel and the fines retrieved by filtering the wash water through a matte filter of the nitrocellulose itself. This procdure not only provides more accurate data for yield determinations, but also insures a more representative final solution. The nitrogen contents given in-the following table were determined .according to methods described in ,A.S.T.M. Specifications, Designation D-301- Grain was determined by means of a spatula, followingthe procedure describedheretofore, wherea fan-shaped drip is produced and a quantative measurement made by looking through this drip. The solutions for'thistest weremade up according to the -method described in A.S.T.M Specifications,:Desighation D'-301f-31T,

Formula-A, for viscosity'determinations; in'some cases where the presence of grain is not readily observed'inthe more dilute solution of Formula laboratory experiments are given below. j A, FOrmula B y be d- Table I r Yield, Moisture Air temp. Relative gms.N/O Exp. No. grains in humidity per gm. Grain Nitrogen Remarks percuJt. wringer percent cotton x linters O, Percent 1 6.88 27.5 60 1.23 Very bad 10.91- -Wringin'g cycle extended to 30'minsfl'to exaggerate I conditions.- Atmospheric air used in this normal v n p p ..tenperature and humidity prevailing during this es n n v v i 2 g 8. 02 25. 5 77 1.40 do 11.24 h 3 min. wringing at prevailingatmosphericconditions. 3 1.48 20. 0 21 1.56 Practlcally none 11.96 Air passed through 3 C8012 bottles and finally through x i concentrated H2804 before entering wringer. o4 -3 39 11,0 77 1.50 do 11. 83. The air in this case was drawn from the outside. Nor- I mal conditions prevailing for this period of the. year. 5 9. 30 29. 0 75 1. 46 Very bad 11'. 73 Air going into wringer artificiallyhumidified by pump 1 v 4 ing it through warmwater just prior to introduction h into wrmger. I. 6 8.06 "29.0 r "65 1.47 do I 11.70 Air sameasin (5). 7 5.0 1.52 Practically none 11. 89 Current of CO: from cylinder passed directly into I g r n wringer. This gas has moisture content oi .0.5 pera can v i 8 1.32 2.0 U 55 I 1.67 .do-; 11.92 Airfrom outside-normal temperature and humidity p for prevailing weather conditions] Inthese experimentsa standard purified cotton linterwas used as the starting material. The linter was dried in an oven to approximately 2% moisture in order to eliminate any variations in results which might be attributed to varying moisture contents. The nitrating acid mix used 'nNos; j Q I. 22.50 'I-I2SO4 I 58.36 r120 19.14

In each experiment 34 grams of the cotton I linter were immersed in 1500 grams of the acid .mix. and agitated. for three minutes, the cellu- R, P. and the charge run for three minutes. The force draft was introduced through the cover of the outside'casing of the separator, carrying The nitrocellulose produced according to .Experiment No. 1 gelled badly in solution, indicating that the nitrocellulose had taken up the sol- .vents but had not dispersed in them. Especially high turbidity was noted and graining was excessive, as was also true of the solution prepared from the nitrocellulose produced according to Experiment No. 2. n

In these experiments the velocity of the air going through the centrifuge was approximately 1.5 to 2.0 cubic feet per minute.--

An additional experiment was conducted on: a plant scale whereby excessive humidity was produced byclosing up the centrifuging. roomand opening steam lines. In this waythe-tmperature was raised to 75 F. and the relative humidity to about 95%.. The moisture content of thefair going into the centrifugev was then about 9.0 grains per cubic foot, the centrifuging being continued for three minutes with air being supplied at a rate of 700 cubic feet per minute. Samples taken from the exposed'surface'of the acid wet nitrocellulose cake at the conclusion'of the centrifugin'g cycle and made up into solutions showed excessive grain. The steam lines were thenclosed and the doors and windows'of the centrifuging room opened to'provide weather conditions prevailing-at that time.

i The temperature dropped to 53F. and'the relative humidity to 55%. The moisture content of the air under these conditions is about 3.4 grains per cubic foot. Samples taken from the exposed surface of the acid wetnitrocellulose cakeat the completion of the centrifuging cycle and made up into solutions showed practically no grain, thus confirming the laboratory scale test.

the centrifuging period 'is'rnaterially decreased or the amount of air supplied to the centrifuge is decreased, or both." Since saturated air at'55 F. contains about 5 grains of air per cubic foot of gas, it will be seen that where the acid wet nitrocellulose is not exposed to temperatures above 55 F. graining difficulties will not be encountered and the present invention is only useful where operations are carried out at temperatures above 55 F.

As previously stated, applicant has definitely found that the graining characteristic of nitrocellulose is directly caused by exposing the acid wet nitrocellulose to moisture, and it naturally follows that by reducing the amount of air passing through the acid wet nitrocellulose there is less opportunity for the nitrocellulose to pick up moisture, and therefore less tendency for the nitrocellulose to show graining characteristics. By altering plant scale equipment to reduce materially the draft necessary to carry off the fumes from the centrifuge, and also by reducing the time of centrifuging, normally in the neighborhood of three minutes, the graining tendency can be reduced, although such means will not eliminate graining where there is a high moisture content in the air. However, by taking such precautions, the graining tendency can be reduced materially eventhough the moisture content of the air be not reduced as low as five grains of water per cubic foot of air. Therefore, the present invention comprises, in a broader phase, the step of reducing the graining characteristic of nitrocellulose, where the humidity of the atmosphere exceeds five grain of water'per cubic foot of air, by at least partially dehumidifying the air supplied to the centrifugal separator, even though the air is not necessarily dehumidified to such an extent that it contains less than five grains of water per cubic foot.

In the experiments described above, the air was dehumidified by passing through calcium chloride bottles and concentrated sulphuric acid, but it is to be understood that on a plant scale various commercial dehumidifying apparatuses would be suitable, such as the silica gel dehumidifiers now used for dehumidifying air in steel plants. Obviously economies in the dehumidifying operation will be effected by reducing the draft of air through the centrifuges, or reducing the time for centrifuging. However, a sufficient draft is necessary to carry off the fumes and incomplete centrifuging may not be economical, due to high acid losses which may occur.

In the foregoing table, it will be noted that not only does the use of a dehumidified gas give a certain method of eliminating the graining characteristic, but also there is less denitration and employing any nitrating acid mix.

an increasedyield. A number of tests show "around -a' 5% increase in the yield where the air contains'less than "5 grains of water per cubic foot, other variablesremaining constant.

The present invention is broadly applicable to nitrocellulose made from any raw 'material and V t The invention is likewise applicable regardless of the method by which the'air or inert gas is dehumidified. This may be done simply by artificially cooling the warm moist air below its dew point to cause pr ecipitationof water before the air is led into the centrifuge.

Particularly on days of abnormally high temperature and humidity it is also advantageous to arrange to use air or other gasof low moisture content in removing acid fumes from the nitrating dippers and, in fact, keeping the acid wet 'nitrocellulo se in arratm'osphere'of dehumidified gas right up to the point where it is drowned in water, although as hereinbefore stated, under most conditions it is sufiicient to dehumidify the air being forced into the wringer.

It Will thus be seen that by the present invention a process has been devised whereby the presence of grain in nitrocellulose solutions may be consistently and entirely eliminated or reduced to a minimum not noticeable in practical uses of nitrocellulose solutions. Furthermore, the present invention provides a process whereby the yields of nitrocellulose may be increased from 2-6% by avoiding losses attendant upon the factors which cause the formation of soluble products in nitrocellulose, characterized by the presence of grain in solutions made from nitrocellulose. Further', denitration tendencies are eliminated or greatlyminimized according to the process of the present invention. By the present invention, manufacturers of nitrocellulose are enabled to consistently produce high quality nitrocellulose throughout the year, whereas heretofore, operating under conditions of high temperature and humidity such as frequently occur during the summer months in the temperate zones, the nitrocellulose varied widely with respect to the graining characteristics and frequently would be. unacceptable to many users of the nitrocellulose. In other words, the present invention provides a process which permits the manufacture of a nitrocellulose of uniform quality, regardless of prevailing weather conditions.

I wish it to be understood that I do not desire to be limited to the exact details shown and described, except as defined in the appended claims, for obvious modifications will occur to a person skilled in the art.

I claim:

1. In the preparation of nitrocellulose where the humidity of the atmosphere exceeds five grains of water per cubic foot of air, said preparation comprising treating cellulose with nitrating acids, centrifuging to separate the nitrating acids, and then drowning the nitrocellulose thus obtained in water, the step which comprises maintaining the nitrocellulose in an inert gas -trifuga1 separator, and then drowning the nitrocellulose in water, the step which comprises sup plying an inert gas having less than five grains of water per cubic foot of gas to the centrifugal separator during the acid separation. l

3. In the preparation of nitrocellulose where the humidity of the atmosphere exceeds five grains of water per cubic foot of air, said preparation comprising treating cellulose with nitrating acids, centrifuging to separate the nitrating acids, and then drowning the nitrocellulose thus obtained in water, the step which comprises maintaining the nitrocellulose in air having less than five grains of water per cubic foot of air during the separation of the nitrating' acids and until the nitrocellulose is drowned in water.

4. In the preparation of nitrocellulose where the humidity of the atmosphere exceeds five grains of water per cubic foot of air, said preparation comprising treating cellulose with nitrating acids, separating the nitrating acids from the thus obtained nitrocellulose by means of a centrifugal separator, and then drowning the nitrocellulose in water, the step which comprises supplying air having less than five grains of water per cubic foot of air to the centrifugal separator during the acid separation.

5. In the preparation of nitrocellulose where the humidity of the atmosphere exceeds five grains of Water per cubic foot of air, said preparation comprisingtreating cellulose with nitrating acids, separating the nitrating acids from the thus obtained nitrocellulose by means of a centrifugal separator, and then drowning the nitrocellulose in water, the step which comprises reducing the graining characteristic of the nitrocellulose by at least partially dehumidifying the'air supplied to the centrifugal separator during the acid separation. I

l 7 PAUL B.-COCHR.AN.

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